1. Field of the Invention
This invention relates generally to a system for treating wastewater containing biodegradeable organic contaminants by oxygenation in contact with active biomass followed by contacting with an activated carbon zone for removal of residual organic contaminants.
2. Description of the Prior Art
A common method for treating wastewater such as municipal sewage or industrial effluents to remove biodegradeable organic contaminants is by the activated sludge process. According to this process, the sewage with or without primary clarification is thoroughly mixed with oxygen-containing gas in the presence of aerobic microorganisms in the activated sludge. The organic matter contained in the water is thereby absorbed and biochemically oxidized by the activated sludge microorganisms. Subsequently the activated sludge is separated, e.g., by gravity settling, and the purified effluent is discharged into a receiving stream or body of water.
While the activated sludge process is one of the most effective and economic wastewater treatment processes available today, it does not achieve complete purification. The effluent, as for example from a municipal activated sludge treatment plant, will contain some oxidizable material including biodegradeable organic matter representing residual biochemical oxygen demand (BOD).
Until comparatively recently, atmospheric air has been employed as the sole source of oxygen in activated sludge plants. In recent years however, this system has been vastly improved by the use of high purity oxygen gas as the oxidant in the manner taught by U.S. Pat. Nos. 3,547,812 to 3,547,815, to J. R. McWhirter et al. In the practice of oxygenation of wastewater as taught by the McWhirter et al. patents, at least one enclosed covered oxygenation chamber is employed wherein the liquid undergoing treatment is intimately contacted in the presence of activated sludge with oxygen-enriched gas from an over-lying gas space to dissolve the oxygen necessary for aerobic biological activity. Such oxygenation systems provide substantial advantages over prior art treatment systems wherein atmospheric air is used as the oxidant in open aeration chambers. For example, the closed chamber oxygenation system is able to operate at biological suspended solids levels several times greater and aeration detention periods several times less than those of air aeration systems while maintaining comparable or better overall levels of treatment. Such advantages are a consequence of the higher mass transfer driving force for oxygen-enriched gas relative to air, which permits higher dissolved oxygen levels to be achieved with economic levels of volumetric oxygen transfer rate per unit of power input. In spite of these advantages however, closed chamber oxygenation systems still produce effluents which contain some small quantity of residual biodegradeable contaminants.
It is known in the art to "polish" or post-treat the effluent from the activated sludge secondary treatment system by contacting the effluent with activated carbon to provide for removal of the residual organic contaminants in the wastewater. Such tertiary treatment has in fact proven effective in providing high overall adsorptive removals of total organic carbon (TOC) and the consistuent biochemical oxygen demand (BOD) from the wastewater due to the morphology of activated carbon which provides an extremely large surface area for physical absorption, e.g., 1200 to 1400 meters.sup.2 per gram of activated carbon.
Recent studies have shown that the absorptive capability of activated carbon for organic contaminants of wastewater can be enhanced by the promotion of aerobic conditions in expanded beds of activated carbon in which biological growth is allowed to develop on the activated carbon surfaces. In a paper by Weber, W. J., Jr., Friedman, L. D., and Blum, R., Jr., entitled "Biologically Extended Physical Chemical Treatment", presented at the Sixth International Conference on Water Pollution Research, Jerusaleum, in 1972, it was reported that an expanded bed activated carbon adsorption system operated under aerobic conditions and treating clarified primary effluent wastewater comprised of approximately 75% domestic waste and 25% industrial waste with a total organic carbon (TOC) concentration in the range of 10-40 milligrams per liter had demonstrated better performance (approximately 15% higher TOC removal) than a corresponding anaerobic activated carbon system operated under the same conditions and had demonstrated a removal capability of nearly 70% by weight adsorption of organic material during nine months of continuous treatment. This performance was markedly superior to that predicted by saturation data obtained from measurements of absorption isotherms and suggested that the observed enchancement of the effective capacity could be attributed to bacteriological activity on the surfaces of the carbon substrate. Subsequent work in the field has borne out the existence of this mechanism as providing an in situ reactivation of the activated carbon adsorbent by biological assimilation of surface absorbed biodegradeable contaminants thereby providing a longer operating life for the adsorbent before saturation occurs and regeneration as for example by thermal reactivation is necessary.
It is an object of the present invention to provide an improved system for treating wastewater containing biodegradeable organic contaminants by oxygenation followed by contacting with an activated carbon zone utilizing the above-described mechanism.
Another object is to provide a system of the above type which is characterized by high oxygen utilization and low power consumption.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.